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For 150 years, terns have always come back to Machias Seal Island, but researchers are trying to find out why they are no longer returning to the tiny island located between New Brunswick and Maine.

Arctic terns and common terns, two species of seabirds that use the island for nesting, have not been using the bird sanctuary since 2006. Researchers are trying to explain why the birds stopped flocking to the Bay of Fundy island.

Tony Diamond, an ornithologist from the University of New Brunswick, said he thinks the reason for the birds abandoning the sanctuary could be one of many reasons.

Researchers are studying why terns are no longer returning to a bird sanctuary on Machias Seal Island, a tiny island in the Bay of Fundy. (Canadian Press)

Diamond said one of the main reasons is their main food sources are diving deeper.

Small herring fish, or sardines, are swimming deeper in the ocean surf, seeking out cooler temperatures in the face of warming currents. This new level for the herring is just under the birds’ surface feeding zones.

"Probably this is related to declining productivity in the Gulf of Maine and Bay of Fundy overall," Diamond said.

"And probably that is related to big oceanographic changes that may be related to climate change. So there's a lot of links in the chain that caused that tern colony to go."

'The sustainability of the whole seabird colony is put at risk.'— Tony Diamond, University of New Brunswick

Less food means weaker birds. Terns tend to be fiercely territorial birds and they will dive-bomb, swarm and defecate on predators or intruders.

Terns are known for fending off gulls, peregrine falcons and bald eagles. Researchers sometimes have to sport hard hats in order to do their work.

But malnourished terns exhibit different behaviours. They will not protect their eggs as aggressively if underfed.

That absence of aggressive protectors means the ecological face of the island will continue to change.

"It means a lot for the other seabirds that nest on the island because there is no longer the squadrons of tern fighters coming in to drive predators away," Diamond said.

"So we're losing a lot more eggs from the puffins and the razorbills that we used too."

"The sustainability of the whole seabird colony is put at risk," he added

Terns tend to have a lifespan of 30 years. Diamond said that while there have been isolated pairs of them return to the island, they've been unsuccessful. Their eggs are eaten by predators within days.

The birds find safety in numbers and it wasn't long ago that terns found Machias Seal Island safe.

"That tern colony was the biggest tern colony in the Gulf of Maine," Diamond said.

"It had two species, common and arctic. It was the biggest arctic tern colony in North America as far as we know, about 2,000 pairs."

The research on the island continues. Diamond’s graduate students spend large portions of the summer months on the island researching and cataloging the remaining puffins, razorbills and other seabirds.

They're often visited by touring birders and other wildlife lovers coming from Maine or Grand Manan.

A lighthouse manned year-round by a Canadian makes up the islands complement.

Machias Seal Island is no stranger to being contested. Located about 20 kilometres off the southern coast of Grand Manan Island and equidistant from the Maine coastline, it remains unclear which country, Canada or the United States, has claim to the 7.6-hectare island.